Artificial line



Sept. 21 1926.

H. w. HlTcHcocK ARTIFICIAL LINE Filed Oct. 20, 1921 2 Sheets-Sheet 1INVENTOR I EWfiQ/afiwa ATTORN EY Patented Sept. 21, 1926.

UNITED STATES may w. nrrcncocx, or NEW YORK, N.'Y., ASSIGNOR To AMERICANTELEPHONE PATENT. OFFICE.

TELEGRAPH COHPAJTY, A CORPORATION OF NEW YORK.

. AB'I'IrIcIAI. LINE.

The princi a1 object of my invention is to provide an e ectrical networkwhich may made closely to simulate a given geographically extendedsignal transmitting line with respect to impedance, attenuation andphase type. Other objects of my invention relate to making the sectionsof properly graded length and electrical characteristics and providingfor adjustment of the several sections to take care of localirregularities in the geographical line. All these objects of myinvention and others will be appreciated upon consideration of aspecific embodiment of its rinciple which. I now'proceed to describe inthe following specification. It will be understood that the scope of myinvention is defined in the appended claims.

Referring to the drawings, Figure 1 is a general diagram, Fig. 2 is amore detailed diagram of a. single section of the artificial line, Fig.3 is a diagram for resolving the currents into their components in thissection, Fig. 4 is a diagram showing the variation of impedance withfrequency, and Fig.- 5 is a diagram showing how the section lengths andcharacteristics are graded along the artificial line.

The special example here presented to illustrate the principle of myinvention is an artificial line designed and constructed to balancea'certain continuously loaded ocean cable of length about 100 nauts(nautical miles) and adapted for simultaneous superposed ordinary andcarrier current telegraph and telephone operation, in which all current.frequencies up to 6,000 cycles per second Application 'flled October 20,1921. Serial No.'508,945.

dicated at 14. This cable 11 is not of uniform physical characterthroughout its length, but its shore ends are specially armored to guardagainst shallow water hazards. Thus it is that the cable consists of aplurality of sections graded slightly in their characteristics from themiddle toward the ends. Moreover, in the manufacture of such a cable, itis commonly made up in sections, each about two nauts long, which arespliced together, and it is impracticable in the manufacture of thesesections to make them iden- .tical in their electrical characteristics.It is among the objects of my invention to provide facility foradjustment of the artificial line to match it to all. suchirregularities. The shore end of the cable 11 with the terminalconductors 12 and 16 are in the cable but .17, where there is located atransformer 13 with a condenser 14 in the coil connected 1 to theconductors 12 and 16. The other coil of the transformer 13 is connectedto the conductor pair 19. Branch conductors 18 for direct currenttelegraphy, with an interposed retardation coil 15 are connected to thei conductors 12 and 16 between. the cable end l rent telegraph channelare connected to the transformer 24, as shown. The frequency range forthis telegraph channel lies above the normal essential voice frequencyrange. These input and output branches 22, 23, 25 and 26 have the usualsuitable filters to separate the frequencies. Other frequency ranges maybe utilized by other hybrid coil transformers in'parallel with 21 and24, with suitable filters in their input and outputconnections. Theparallel branches of the conductor pair 19, throughthe respectivetransformers 21 and 24, are united and lead through the network 27,which balances'tho part of the conductors 19 in the cable 20;

33 balances the part of the conductors 18 within the cable 20. A channelfor ordinary duplex direct current telegraphy is connect- I ed to theterminals 31.

It will be seen that the apparatus at the cable hut 17 is balanced bythe transformer 28, and the terminals 39 and 40 correspond respectivelyto 12 and 16. Connected to these terminals 39 and 40 is my, improvedartificial line 43'which balances the cable 11.

This artificial line 43 consists of a large number of sections 35connected in serial order. These sections are alike in general plan butdiffer in equivalent length and in ohmic resistance as will be pointedout presently. Each section 35'between the two terminals 39 and 40 onone end and 41 and 42 on the other end consists of a resistance 36 andinductance 37 in series on one side,

a like series combination of resistance 36' and inductance 37 on theother side, and a pair of equal cross-connected condensers 38.

The resistances 36' and inductances 37 are adjustable as indicated inFig. 2.

When the resistances 36,. inductances 37 and the capacities 38 of thesection of Fig.' 2 are given properv values, determined by.

the series resistance and inductance of the geographicahline and itsdistributed shunt capacity, then the artificial line section ispractically equivalent to a length of the geographical line. Theseelectrical characteristics are chosen to make the equivalent lengths ofthe sections comparatively short at the end near the ofiice and not soshort at the remote end The exact dimensions in thisillustra-tiveexample of my invention are indicated in Figs. 5, which shows 20halfnaut sections next to the office end, then 20 one-naut sections,followed by longer sections until the remote section with an equivalentlength of twelve nauts is reached.

Assuming for the conductor pair of the geographical line that the ohmicresistance per nautis R ohms, inductance per naut is L henrys, anddistributed shunt capacity per naut is C farads, if we make the networkof Fig. 250 that each resistance 36 .is

-38 is 30, where is any number, then this network interposed in thegeographical line is equivalent in impedance to a length of n v Thedefinition of characteristic impedance assumes an infinite line, andaccordingly the impedance across 39, 40 is the same as across 41, 42, asindicated by the'chal'acter Z in Fig. 2. Let the impedance of branch 39-41 be and let the impedance of branch 3942 be i where Z and Z arerespectively the series and shunt impedances per unit length of thesmoothdine. Solving the network of Fig. 2 by the ordinary \Vheatstonebridge formula, the result is obtained that The last expression is thefamiliar formula for the characteristic impedance of the smoothgeographical line whose constants are R,L and C. Thus it will be seenthat the characteristic impedance is the same whether the sections ofFig. 2 are great or small in equivalent length. The advantages of makingthem small will be pointed out presently, and itwill be shown that theyshouldbe shorter near the office end. 7 As to the propagation constant,we know that for the smooth line, it is given by where P is defined bythe equation 6P:I1/I2 and where 1 /1 is the ratio of the currents at theinput and output ends of a unit length of the smooth line.

Let the input and output currents for the section of Fig. 2 be .I, and1,. By the symmetry of the structure. it. is seen that the currentsthrough the branches 3942 and 41-40 are equal. Call these I. Resolvingthe currents in the network of Fig. 2 as shown in Fig. 3, we have byKirchoifs law,

I1IZ/ IIIZ2I+(I!I+I2')Z1I and.

Eliminating IQ", substituting I 'i for Z" and simplifying, we get 1 E 1/2' 1/ T By analogy to the definition of P, let the foregoing expressionequal e By definitIOII' of the hyperbolic tangent, tanh Substituting andreducing,we get By a simple transformation we get I I I When 5,- issmall, tanh and are nearly equal. Hence we see that by making n small wemake P and n .to be nearly equal, and thus we make the propagationconstant of the section of Fig. 2 to be nearly the same as 12 units oflength of the smooth line. Accordingly the size of the sections isdetermined largely by the consideration that they should be made'smallenough to secure asufiiciently approximate equality of the propagationconstant per; section with the corresponding length of the smooth line.The specific application of this principle in the present case' isexhibited in Fig. 5 where the gradation of section length is shown byappropriate legends on the drawing.

If the geographical line were perfectly smooth, the resistance andnegative reactance components of its characteristic im pedance wouldvary with frequency some what according to the smooth curves of Fig.

4. Due to the inevitable slightirregulari-.

ties in the geographical line that have already been mentioned, a testof thecharacteristic impedance for varying frequencies gives a plot thatmay depart from the smooth curves, for example, somewhat as indicated bythe dotted curves of Fig. 4. These departures are due to partialreflections of current waves from the junction points of sections ofdiffering characteristics. They may be analyzed by graphical oranalytical methods, and the actual distance of the irregularities fromthe .cable but may be determined in this way, and thereupon adjustmentsmay be made in the proper artificial line sections to matclrtheseirregularities. These adjustments will be made when the cable is startedin operation and ,will be made once for all, because the irregularitiesin the geographical line are permanent, and therefore the correspondingadjustments of the artificial line are equally permanent.

For high frequencies, as already stated,

the sections should be small, in order to make the propagation constantnearly equal to that for the geographical line. But the high frequencycurrents are most rap dly attenuated, and for this reason the sectionsmay be made longer at the distant end of the artificial line. Alsobecause. the current is far less attenuated in theiiear sections than inthe remote sections, much finer adjustment may be. necessary in the nearsec tions.

current is so highly attenuated that, even if a considerable part of itwere reflected back to the oflice, its effect there would scarcely benoticed. This is true for higher frequencies, say above 300 cycles persecond, though lower frequencies would not be so much at-' 'tenuated andmight be reflected back and At the remote end of the artificial line the1 noticed atthe otlice- The artificial line in terminated at its remoteend by a network 44,consisting of a series resistance 45 shunt edby acondenser. 46 and series resistance 47. The impedance of thiscombination approximates sufliciently to the impedance of the distantterminal apparatus when look ing into it from along the actual line.

In the discussion hitherto it has been tacitly assumed that theseries-resistance R of the geographical line is constant, but in fact itis a'variable function of frequency.

For low frequencies the return current spreads out in the sea water,whereas for high frequencies it is largely confined to the cable sheath.thus increasing the circuit re- 'sistance.

Moreover the inductive loading involves more or less eddy current andhysteresis loss, which are functions of frequency and increase withfrequency and therefore increase the apparent resistance to the currentflow in the signaling circuit. It is impractical to construct'theartificial line to make itsresistance vary correspondingly. It isnecessary to make the direct current resistances of the artificial lineequal the cable resistance 'at about 1000 cycles per second. This is toohigh for direct current and therefore the resistance is reduced for thesections farther out, so that the total direct current resistance of thecable has the proper value. i a

In the foregoing specification, I have disclosed a network with sectionsof different equivalent length; by this term I mean the length of theextended lineto which the network section is approximately equivalent inits transmission properties. A. network of short equivalent length isone which is equivalentfto a short length of the extended line. y

I iclaim t 1. An artificial line consisting of sections in serial order,each intermediate section being connected with the adjacent section onone side at two points and with the adjacent section at the other sideat two points, impedances connecting each point on one side with eachpoint on the other side, one

-pair of non-adjacent impedances consisting each of aresistance and aninductance in series and the remaining pair of impedances consistingeach of interposed condensers.

2. An artificial line of sections in serial order, each se' tion being abridge type network, the sections near the input end being made smallenough in equivalent length so that the propagation constant per sectlonshall be approximately the same as its limiting value when the size ofthe sections is made indefinitely small in equivalent length.

3. An artificial line of sections in serial order, each section being abridge type network with the impedancevalues of its arms so relatedto-the characteristirs of a given smooth line that the sections simulatedefinite lengths of that line for characteristic impedance. the.sections being'made small enough in equivalent length so that thepropagation constant per section shall be approximately the same as forthe corresponding length of the smooth line.

4. An artificial ,line of sections in serial order, each section being abridge type network. the sections being small in equivalent length andeach section being matched closelv to a defin te finite length of agiven smooth line. 5. An artificial line of sections in serial order,each section be nga bridge type network. the sections being small inequivalent length at the input end and in reasing in size awaytherefrom, and each section being closelv matched to a definitecorrespond .are alike, one pair of like lmpedances each consisting of aresistance and an lnductance ing finite length of a given smooth line.

6. -\n artificial line of sections in ser al order. each section being abridge type networkbetween two pairs of terminals, each pair ofterminals on one side being conneeted to a pair of the next succeedlngsection on that side, and having a pair of its arms of adjustableimpedance whereby they can be matched to local irregularities in anactual smooth line simulated by the artifici l line.

conductor, and an artificial line to balance it, said line consisting ofbridge type net- 7. in combination, a submarine signaling 9; Anartificial line of sections in serial order, each section being a.bridge type network liaving the series impedance of each arm of one pairof arms equal to half the series impedance of a definite length of agiven smooth line and having the admittance of each arm of the otherpair of arms equal to half the shunt admittance of the same length ofthe given smooth line, the equivalent lengths of said sections beinggraded in decreasingmagnitude toward the inputend.

10. In combination. a loaded signaling condu tor and an artificial lineto balance it, said line-consisting of sections in serial order, thesections near the input end being made short enough in their equivalentlength so that the propagation constant per section shall beapproximately the same as its limit ing value when the size of thesections is made indefinitely short in equivalent length;

11. An artificial line of sections in serial ordexnrach intermediatesection being conne ted at two points with the adjacent section on oneside and at two other points with the adjacent section on the otherside, the two points' on one side being connected with the two points onthe other side by four impedances of which two non-adjacent impedancesare alike and the remaining two in series and the reinaining pair eachcons sting of capacities.

said lire consisting of sections-in serial order te made-short enough intheir equivalent length so that the propagation constant per sectionshall be approximately the same as its limitingyahie when the size ofthe sections is made indefinitely short in equivalent length, and thesections farther away from the input end being made longer in equiva--lent length. i

V In testimony whereof, I have signed my d name to this specificationthis 18th day of October, 1921.

HARRYWV. HITCHCOCK.

sections near the input end being

